The most common form of volumetric residential gas meter is the diaphragm gas meter. This is a mechanical device working on the positive displacement principle, allowing a fixed volume of gas through per complete cycle. Mechanical meters are subject to wear in normal operation, which leads to increasing inaccuracy with time, and the eventual possibility of complete failure. The increasing prevalence of automatic meter reading (AMR) means that very often some form of encoder must be interfaced to the mechanical readout, in order to be able to read the consumption information automatically.
It is desirable to provide a gas meter that contains no moving parts, i.e. a static gas meter, in which a measurement of the volume of gas consumed is available directly in an electronic form. Other benefits follow from such an implementation, including the ability to set more complex tariffs based on time of use, peak demand, or local variations in gas pricing, or the ability to share information with other residential energy sources such as electricity, oil or renewable energy sources.
Three types of static volumetric gas meters have been developed. The first is the ultrasonic time-of-flight meter, which is available commercially for niche applications that can bear the high cost of this kind of meter. The second known technology is the thermal mass-flow meter, which is a relatively new addition to the field, and uses a bypass method and a micro-machined sensor. The third type is a fluidic oscillator meter, which was developed in the 1950's. All of these metering technologies share the disadvantage that they are more expensive than mechanical meters, and require significant battery power, which also increases the cost.
U.S. Pat. No. 3,688,106 (Brain) describes a meter for measuring the velocity of gas in a duct. The meter has an ion source and two ion collectors, so that gas in the duct is first ionized and then passes the collectors. A voltage pulse is applied to the first collector and the interval between this pulse and the resulting effect in the number of ions collected at the second collector is measured to give gas velocity. Gas density is measured by determining the number of ions collected between pulses at the second collector, and mass flow is obtained from the product of velocity and density. In this system, the voltage pulse applied to the first collector is a 100 Hz square wave and a voltage of 120 volts is applied across the second collector. The high voltage and high modulation frequency make this design unsuitable for low-voltage battery-powered operation required by a domestic gas meter. Other configurations of ionisation velocity gas meters are described in U.S. Pat. No. 3,842,670 and U.S. Pat. No. 2,632,326.
It would be desirable to provide a gas meter of the general type described in U.S. Pat. No. 3,688,106 (Brain), which would be capable of functioning with an operating voltage of a few volts, so that the meter could be powered economically by standard batteries. With the metering geometry described by Brain, however, it is essential that the electrodes of the collectors are spaced sufficiently that the collectors present little or no impedance to gas flow. Thus, an operating voltage in excess of one hundred volts is required to provide a sufficiently large electric field at the collectors for the meter to function. For the same electric field to be generated with an operating voltage of only a few volts, the duct in which the Brain meter is mounted would need to be one hundred times smaller in diameter, which would significantly impede the flow of a domestic gas supply.
This invention, at least in its preferred embodiments, seeks to provide an improved volumetric gas meter operating on the principle of the electrical manipulation and detection of an ionised gas stream, using the underlying principle that the velocity field of the gas interacts with the ionisation distribution, and alters the detected signals. In particular embodiments, the gas meter is especially suitable for metering of gas usage from a national or regional supply network.